This invention relates to high voltage direct current power transmission systems and more particularly to an inverter firing control for such systems.
High voltage direct current (HVDC) power transmission systems are commonly employed for interconnecting high voltage AC networks or a distant generating station to an AC network. Such systems typically consist of two converter stations interconnected by a transmission line or cable. At the generating or transmitting end, the converter comprises a rectifier for rectifying the alternating power to direct current and voltage while at the receiving end the converter comprises an inverter which transfers power from the DC transmission line to the AC network.
A typical HVDC system may include a bipolar transmission line with the converters each comprising a pair of series connected three phase, two-way six pulse bridges which include thyristor or mercury arc valves. Such valve bridges permit the conversion of three phase alternating voltage and current to direct voltage and current or the inversion of direct voltage and current to three phase alternating voltage and current. Valves, such as thyristors, conduct current only in the forward direction from anode to cathode and only when the forward voltage across the valve is positive and the valve receives a control pulse. Once the valve has started to conduct, the magnitude of the current is determined solely by the main circuits outside the valve and is not influenced by a negative gate pulse. The flow of current through the valve continues until it decreases as a result of external influences and attempts to become negative. Reverse current flow is prevented because the valve would be reverse biased so that the current through it is extinguished. In the forward direction, the valve will block current flow until a control pulse is applied to the gate. As a result of these properties, the operating cycle of a valve is divided into a forward blocking interval, a conducting interval and a reverse blocking interval.
In a three phase, two-way twelve pulse system, each phase of the transmitting and receiving AC networks is connected to the positive and negative conductors by two pair of valves oriented in the forward direction. The valves are actuated by a firing control system which provides gate signals to the valves in a predetermined time sequence to effect current transfer or commutation from phase to phase.
When the valves are operated in the inverter mode, the direct voltage is negative when referred to current direction. This means that the voltage across the valves is positive most of the time. To establish a forward blocking voltage, the charge established during the conducting period must be removed. Therefore, the valve requires a time interval with a negative valve voltage between the end of the conducting period and the application of positive voltage. The electrical angle corresponding to this time period is called the margin of commutation or the extinction angle.
In typical inverter operation, with one valve conducting, the firing of the next succeeding valve is ordered in sufficient time before the next zero crossing, at which time the phase-to-phase voltage will become positive. Thus, the commutation from the off-going valve to the on-going valve must be finalized in time to insure a sufficient commutation margin. If for some reason commutation is not finished when the voltage across the off-going valve become positive or the commutation margin is so small that the valve does not have time to regain sufficient forward blocking capability, there is a transient disturbance in the inverter operation known as commutation failure.
As noted above, to establish the forward blocking capability of a valve, the charges established during the conduction interval are removed by providing a negative valve voltage for a time interval corresponding to the commutation time. Since rectifiers are normally operated at firing angles of less than ninety electrical degrees, this represents no problem in rectifier operation. However, such commutation failures are a concern with inverter operation because of the desirability of maintaining the extinction angle as small as possible to maximize power transfer. Conventional inverter firing angle control systems normally attempt to prevent commutation failure by measuring the time difference between the end of valve conduction and the time of the previous voltage wave form zero crossing. This permits the continous prediction of the minimum extinction angle. However, certain types of disturbances, such as single phase unbalances in the AC network, cause commutation failures to occur approximately ten milliseconds after the beginning of the disturbance. This results in a modification of the voltage wave so that information regarding previous zero crossings is no longer valid for predicting the extinction angle necessary to prevent commutation failure.